This invention relates generally to the manufacture of generators and, specifically, to improving the joint between a hydraulic header clip or fitting and a liquid cooled armature bar.
The armature windings of large steam-turbine generators are generally water-cooled. The armature windings are composed of half coils or armature bars connected at each end through copper or stainless steel fittings and water-cooled connection rings to form continuous hydraulic circuits. The hydraulic winding circuits are typically connected to inlet and outlet water manifolds with plastic hoses that provide electrical isolation. The manifolds are connected to the stator water cooling system which cools, filters and deionizes the water and pumps the water back to the armature winding. The armature bars are composed of rectangular copper strands arranged in rectangular bundles. All of the strands may be hollow, or the bundle may include a mixture of solid and hollow strands. The hollow stands thus have a duct for conducting cooling water. A braze alloy joins the strands to each other and to a waterbox header clip at each end of one of the armature bars. The clip functions to deliver water to and collect water from the hollow strands. The clip is connected through copper or stainless steel fittings to a second armature bar to form a complete armature coil element of the winding.
The braze joints between the strands and between the strands and the clip must retain hydraulic and electrical integrity for the expected lifetime of the winding. The braze alloy or filler metal that joins the strands and the strands to the clip is currently a self-fluxing copper-phosphorous alloy, also referred to as a BCuP alloy. This family of alloys typically contains 3½–7 weight percent phosphorous. The surface of the braze joint is constantly exposed to a deionized, oxygenated water environment.
It has become evident that the factors required to support a crevice corrosion mechanism in a clip-to-strand braze joint are phosphorous, copper, suitable corrosion initiation sites and water. If any one of these factors are eliminated, crevice corrosion should not occur. Corrosion of the phosphorous-containing braze alloy and adjoining copper strand surfaces can occur under certain conditions, specifically if critical crevice geometry and crevice water chemistry requirements are met. The corrosion process can initiate if the joint surface contains any surface crevices, pinholes, or porosity at or near the surface of the joint, and if the critical water chemistry conditions that can support corrosion develops within these sites. The corrosion process can progress through the braze joint as long as critical crevice geometry and water chemistry conditions exist. In addition, porosity within the braze joint can accelerate the total apparent corrosion rate. Eventually, the path of corrosion can result in a water leak through the entire effective braze joint length, compromising the hydraulic integrity of the clip-to-strand joint.
U.S. Pat. No. 5,796,189 discloses an arrangement where all of the strands are cut to the same length and the copper-phosphorous (BCuP) braze alloy is pre-placed flush to the ends of the strands. A braze alloy anti-wetting agent is used on the ends of the hollow strands to prevent plugging of the hollow strands and an inert purge gas is used during the brazing cycle. Use of the anti-wetting agent, although effective for preventing hollow strand plugging, can result in strand faying surface contamination and a poor effective braze joint length.
A recent pending and commonly owned patent application Ser. No. 10/418,296 filed Apr. 18, 2003, teaches the use of extended hollow strands in combination with the use of a de-oxidizing gas as the purge gas. The extended hollow strands eliminate the need to use an anti-wetting agent on the ends of the hollow strands and the de-oxidizing purge gas limits oxidation during the brazing cycle, de-oxidizes strand surfaces and the braze alloy prior to alloy melting, and improves braze alloy wetting and flow. However, this practice continues the use of a phosphorous-containing braze alloy and, therefore, the potential for crevice corrosion still exists, although it is greatly minimized due to the significant reduction, or the elimination in most cases, of surface initiation sites.